365 



of the focusing glass, the water-mill was turned on and the dark 

 slide containing the sensitive collodion plate was substituted for the 

 ground glass. Mr. Hartnup then took his station at the finder, and, 

 with a tangent rod in each hand, by a steady and continuous move- 

 ment, kept the point of intersection of the cross wires stationary on 

 one spot on the moon's surface. 



When the motion was most perfectly neutralized, I uncovered the 

 sensitive plate at a given signal and exposed it, counting the seconds 

 by means of a loud- ticking chronometer by my side. 



From the ease with which on my first attempt I could keep the 

 cross wires of the finder fixed on one point of the moon by means 

 of the tangent rods, I confidently believe that with the well-tutored 

 hands and consummate skill which guided this noble instrument, the 

 moon's image was as motionless on the collodion plate as it could 

 have been were it a terrestrial object. 



The negatives which I obtained by these means were exquisitely 

 beautiful, and so minute that I could not obtain paper with a suffi- 

 ciently fine surface whereon to print copies which would do them 

 justice. It was evident that they would bear magnifying several 

 diameters and still remain sharply defined. The expense of carrying 

 out this design here stopped me, when by the kind advice of Pro- 

 fessor "Wheatstone I applied to the Royal Society, whose munificence 

 has so frequently been the cause of bringing to a successful termina- 

 tion investigations of the highest importance. 



A half-plate photographic combination of lenses, by Ross, was 

 screwed the reverse way into a large sliding camera body 10 inches 

 high by 11 inches wide, and capable of sliding from 18 inches to 

 3 feet long. At the end of the sliding body opposite to the lens, was 

 a groove to admit either a focusing glass or a dark slide for the 

 sensitive plate. A smaller camera body was screwed into the other 

 end of the brasswork of the lens, having also a groove in front to 

 admit of a sliding box capable of holding the small negatives. A re- 

 flector was placed in front of all, so arranged as to move in altitude 

 round a centre, and, being fixed in any required position, to reflect 

 the diffused light of the sky through the negative and lens parallel 

 with the axis of the latter. 



Preliminary trials showed me that there was no good gained by 

 magnifying the small pictures more than about 20 times, as after 



